Burning Issues

The problems caused by rings and build-ups in a kiln system always create turmoil and frequently a loss of production. Most of these problems can be controlled if not eliminated.

The picture below, indicates the major causes of ring and build-up. There must be a good evaluation programme, which includes a review of the Literature. When this accomplished, there are definitely solutions to the problems of ring and build-up in the kiln. Frequently the solution requires forgetting Some preconceived ideas.

This section will not cover all of the ring and build-ups that can occur, but will Address those most frequently encountered. There are problems  associated  with the burning of waste fuel which can attributed to flame position, alkalis, chlorides and Sulphur.

Major Causes of Ring and Build-Up

1.     Overheating

2.     Slow clinker quench

3.     Fuel impingement on the burning zone

4.     Long flame

5.     Chlorides

6.     Sulphur

7.     Potassium

8.     Mechanical restrictions

Evaluation Programme.

1.     Sampling

2.     Care of sample (temperature, air and moisture considerations)

3.     Samples which are consistent and representative

4.     Documentation of condition before and during time of the problem

Solutions

1.     After raw materials and fuel

2.     Control internal alkali, Sulphur and chloride cycle.

a.   Install a kiln gas by-pass for preheater and calciner kiln.

b.   Do not return as much total dust especially where precipitator fields discharge to individual conveyors.

c.    Determine time cycle for build-up. Adjust kiln burner (permit clinker quench, shorten burning zone length, eliminate fuel impingement on the load, locate burner on kiln center line end slope).

3.     Adjust kiln material and gas temperature profile.

4.     Install kiln internal restrictions such as dams or orifice rings.

5.     Maintain the secondary air temperature consistently.

Figure 2. Show a basic understanding of air volume changes attributable to changes in temperature. Our experience indicates that many people tend to forget this relation. They comment "I didn’t increase the air flow,” the flame look like it is on the load. lt wasn’t yesterday; someone must have moved the burner.''

Figure 2. Shows the increase in volume caused by temperature changes. One cubic foot of air at 100ºF weighs about 0.071 pounds. The weight of one cubic foot of air seems insignificant, but at each of these plotting the weight of air is the same (0.071 pounds), only the  volume has changed. 4.4 cubic feet of air at 2000 ºF   still weigh   0 .071 pounds.

Fig 2

We often hear the question, "Where is the best place to sample the kiln discharge hood pressure?" But the real question is "where on the hood does the sample point (or points) give a pressure reading that permits relative control?" That is, where are the conditions today similar to what they were yesterday? The next question is "what is the correct kiln discharge hood pressure?" The kiln discharge hood should be at a slightly negative pressure to permit observation by instruments or persons with­ out undue overheating and dusty conditions. From the standpoints of good housekeeping and maintenance, the hood pressure should be slightly negative. This value should be determined by trial and error for each system. It is always advisable periodically to review the selected set point to determine if conditions have changed.

Once the desired kiln discharge hood press is selected, that is the target, whether it is 0.05 inches w.g., -0.01, -0.1, -0.15 etc. Quite possibly the most serious effect on hood pressure sampling over the years has been our attempt to "bum on the nose." All changes in fuel ignition are immediately detected by hood pressure changes-there is no dampening effect as there is when the flame is away from the nose.

Figure 3 shows the different pressure conditions found in the kiln discharge hood. This is why more than one sample point is needed, with all Manifold together to serve as one sample source. The result is the measurement of an average pressure.

It is interesting to note the effect higher secondary air temperatures have on the kiln discharge hood pressure. The increase of secondary air temperature increases the volume of air as well as the velocity. This increase of velocity tends to drive the secondary air and dust toward the top of the hood. This condition always creates a dusting and puffing at the top of the hood over the kiln, whereas the bottom side of the kiln may be at slightly negative pressure.

Figura 3.

A change in secondary air temperature can move the flame position up or down. Certainly, a change of secondary air temperature wilt alter the fuel ignition rate, but the concern in this example is the positioning of secondary air temperature is increased. Velocity through the cooler throat increases to 1275 feet per minute. This increase of velocity tends to raise the flame path. which usually causes the burning zone to cool off and the calcined material to flush into the burning zone.

These examples show why it is more important to maintain a constant secondary air temperature than to attempt to reach the highest possible temperature.

Figure 4. Shows the burner positioned on the kiln Centre line and slope. This position has been adjusted during operation to compensate for the secondary air 's tendency to lift the flame path ln this example the intent is to direct the flame tip on the kiln Centre line and slope. Figure 4 indicates that the average secondary air temperature is 1000 ºF. The volume of secondary air passing though the fixed throat area has a velocity of 900 feet per minute.

The system in figure 5 is identical to that in figure 4 except that the secondary air temperature has been reduced to 700ºF. The velocity of the secondary air through the clinker throat has now been reduced to 715 feet per minute. The Flame path has been lowered and the tip is no longer on the kiln Centre line and slope. resulting in fuel impingement on the load. The problem of fuel impingement on the load is definitely more pronounced when the burner is adjusted to run the flame toward the load. Microscopic analyses often indicate that the clinker was produced in a reducing atmosphere on this date, whereas the day before this was not the case.

Figure 6. Shows what happens when the normally, attempts to achieve the maximum secondary air temperature produce cyclical operation of the kiln. This promotes the production of clinker burned in a reducing atmosphere, slow quench of the clinker mineral dusting of the kiln discharge hood and kiln ring formation.

It is also important to recognize that the secondary recorded by most plants is a relative temperature. The secondary air temperature is usually detected by placing thermocouple somewhere near the clinker cooler throat area.

The value indicated by this method of sensing not only measures the air temperature, but it also detects radiated heat from the clinker and the flame. A true secondary air temperature is measured by aspirating a portion of a secondary air away from the clinker and past a thermocouple sensor. 

Methods for aspirating air from the clinker cooler have proved to be impractical primarily because of wear created by the clinker dust. Quite possibly the calciner kiln system permits the most accurate measurement of combustion air temperature. The calciner kiln system aspirates combustion air from the clinker cooler as tertiary air for the calciner.

In spite of its inaccuracy. the thermocouple placed in the clinker cooler throat has been accepted as indicating a usable relative secondary air temperature for day -to -day kiln operation. This method of detecting secondary air temperature is fine if we remember that It is a relative temperature and may read much higher because of radiated heat from the clinker.

lt may increase or decrease. depending upon changes in the clinker cooler bed. without much real change in air temperature. 

Fluctuation of secondary air temperature is one of major causes of ring of rings and build- ups. The kiln flame and location must be controlled to maintain a stable operation. A stable kiln operation should create the patten of coating and a ring formation show in figure 7. This drawing show only a small amount or no coating from the burning zone to the kiln discharge end. The ring that forms 80 – 115 feet from the kiln discharge end is in the area where is complete and the liquids begin to from the location of this ring depends upon the burning zone length it is formed because of the coexistence of calcined material, a small amount of liquid, and material still in the solid phase. This create prime conditions for build-up. The ring does not adhere to the refractory, is not dense and it, very fragile. lt breaks up and falls out when the kiln temperature is changed by alteration to de calcining zone and material preparation. It may fall out when flame length and location change.

This ring is regarded as an asset because it serves as an orifice that increases the gas velocity at its location. This tends to hold back and mix aerated material. While the ring is present kiln operation tends to be stable. with less material flushing into the burning zone. If all conditions remain stable, the ring remains and assists operation. It does not grow substantially ass the stable operating time increase. For several days often the ring falls out, kiln operation may be cyclic and it is difficult to keep the raw load out of the burning zone. We have experienced stable operation with the flame directed toward the load, and this location may be satisfactory as long as the flame tip is not on the load. However, when the flame tip is directed into the load. Any change such as decrease in secondary air temperature. May create the condition where the flame tip is projected through the load (fig 9). This lengthening of the flame causes fuel impingement on the load, but also causes the conical long flame ring build-up show figure 8. When this ring is detected, it can be broken up and dropped out by shortening the flame. This type of ring can also be prevented with a short flame with its tip directed on the kiln Centre line and slope.

Another example of a long flame is show in fig 10. In this case the flame tip is at least directed on the kiln slope and parallel to the kiln Centre line. There are apparently sufficient liquids available to produce a sticky environment of a material ball. Ball which are 6 to 12 inches in diameter have been found in the middle of the calcining zone. A few of these balls grow to diameters of 6-8 feet. The larger ball look alarming when they are first seen passing through the burning zone. Burning with a shorter flame length prevents additional balls forming unless they are caused by a high concentration of alkalis, Sulphur, and chlorine.

The nose ring (fig 11) has been described as an “some kilns operate with a nose ring most of the time. This tend to restrict clinker discharge from the kiln. Microscopic evaluations of clinker produced during the presence of a nose ring indicate the presence of slow quench. The nose ring permits a very slow quench. The nose ring permits a very slow quench of the clinker because the material is pooled when it passes out of the burning zone. Quick quench of the clinker mineral must be completed within the kiln or it will not be achieved. 

Slowly quenched clinker causes the C₃S to revert back to C₂S and free lime. Further slow cooling causes the C₂S fin the beta state or high-temperature form to change to the gamma state of C₂S (a low temperature form). The gamma form of C₂S is a dust and no longer forms a nodule. This dust is picked up by the flow of air and carried back into the kiln where it enters the burning cycle again. The slow quench cycle continues as long as the nose ring persists to act as a dam. The suspended particles returning with combustion air are easily preheated because the surface area is maximized. The liquid available at the kiln nose permits its adherence of the dust particles, and the building of the nose ring continues. 

Figure 12 shows an example of a snowman on the clinker cooler back wall. Some Snowmen grow tall enough to reach the burner pipe. Generally, the larger the kiln, the larger the snowmen. Depending upon the installation procedure of refractory over dead grates, some kiln systems

Form snowmen on the clinker cooler side wall near the throat. The snowmen build-up is caused by the same problem that promotes the nose ring build – up—that is, slow quench of the clinker.

Microscopic evaluation of clinker shows whether the material was slowly or quickly quenched and whether C₂S changed from the beta to the gamma state. Both the nose ring problem and the snowmen build-up can be eliminated by adjusting the kiln burning operation so that the clinker is quickly quenched within the kiln. We have learned to live with a dusty kiln discharge hood, especially in larger kilns. The old small wet-process kilns were seldom dusty because the fuel consumption was high and we could not gain quick enough ignition to burn on the nose. This promoted the quick quench of clinker within the kiln. We also found that the old wet kiln produced the most reactive clinker, which permitted a lower fineness for similar compressive strength levels.

Our most recent experience of putting this flame technology into practice was with a large wet kiln, it was necessary to remove large snowmen from the clinker back wall. These snowmen were giants, 10-12 feet high and 6-8 feet in diameter at the bottom. The kiln discharge hood was so dusty that we could not see the nose of the kiln. The nose refractory had to be replaced every six months and the nose castings every 12 months. The kiln burner was adjusted to shorten the flame: This reduced the burning zone by about 45 per cent. The burning zone temperature increased from 2600 F to 2750 F and NOx fell from 750 ppm to 350 ppm. The clinker went from slowly quenched to quickly quenched. The clinker cooler snowmen were eliminated, the kiln discharge hood cleared and we could see the flame and burning zone. In addition, the 28 day compressive strengths increased by 600 psi over a 90-day period without any increase of fineness.

Figure 13. Displays a ring formation which occurs in the calcining zone or the area where the gas temperature is sufficiently low to permit condensation of Sulphur and chloride compounds. This ring is a part of the alkali, Sulphur, and chloride cycle. All kilns have a variety of ring in this area: some consist of a small amount of punky coating with larger rings (fig 13) the kiln has to be shutdown to physically remove the build-up. The elimination of the cause normally requires a fuel change, such as a lower Sulphur fuel, and the return off less kiln dust. If a microscopic evaluation of the clinker indicates production in a reducing atmosphere, the burner should be adjusted to eliminated fuel impingement on the load. This will permit a higher clinker Sulphur level which removes a similar amount from the cycle. If the long wet and dry kilns use an electrostatic precipitator, the dust collected in the final fields can be wasted as high alkali, Sulphur, and chloride material.

The electrostatic precipitator works well as a kiln gas bypass system for the long wet and dry kiln systems. Since the solidified alkali, Sulphur, and chloride particles are very small, they are concentrated in the final field of the precipitator and are easily separated and removed from the system.

Figure 14. Show some areas in the suspension preheater where problem build-ups often occur. As we proceed up the preheater in the direction of the kiln gas flow. The first problem area is at the kiln feed shelf. This problem on a preheater kiln is either caused by operating with a high level of carbon monoxide in the exit gas. Ambient air leakage causes a localized condensation of alkali, Sulphur, and chloride compounds. These chemicals are vaporized in the burning zone and exit as a kiln gas until temperature conditions are sufficiently low (about 1800 F) To cause condensation to the liquid state. Normally, the preheater kiln exit gas temperature is above the condensation point. When ambient air leaks into the kiln feed end housing there is a localized cooling of the kiln gas at the leakage source that results in build-up at that point. A different type of calciner kiln system build-up at the feed shelf and feed end housing walls can also be caused by leakage air. This build-up is caused when the kiln feed is nearly calcined and there are C4AF liquids present. However, if the gas dust concentration is sufficiently high, the liquid will adhere to the dust particle rather than to the surface of the wall, thereby preventing a build-up.

This situation can be artificially duplicated by the introduction of dust from the stage III cyclone material discharge and/or creating a rough feed shelf surface which causes a splashing of the feed out into the gas stream. Dust re-entrained in the kiln exit gas bypass system, so the feed shelf must have a smooth surface when running a kiln gas bypass system.

Figure 15 shows a build-up above the kiln gas bypass take-off and within the quench chamber. The build -up in the kiln rise above the kiln gas bypass take-off is caused by the leakage of quench air from the quench chamber. Proper sizing of the bypass quench chamber inlet can ensure that quench air does not enter the riser duct.

The example in fig 16 show the parameters used for design and adjustment of the quench chamber inlet. A two*inch pressure loss through the quench chamber inlet will also ensure that the quench air exits to the kiln by pass induced draught fan.

Alkali, Sulphur and  chloride  compounds create no build-up problems if they exist in either the gaseous state or the solid state. However, if  they exist in the liquid state, they  behave like water on dust. The secret to efficient kiln gas bypass system operation is taking a portion of the kiln exit gas at plus 1900 ºF and instantaneously quenching it to about 750ºF. This permits the alkali, Sulphur and chloride compounds to pass from the gaseous state directly to the solid state without passing through the liquid state. Some designers and operators quench to higher temperature levels, i.e. 900ºF to 1100ºF. Our experience has found more potential for build-ups in the quench chamber at these higher temperatures.

The kiln gas bypass system appears to work best when the quench chamber and kiln riser duct take-off are placed above the kiln. As the gas and dust exit the kiln, the dust is thrown against the feed shelf while the gas is turned upward. This separates dust particles from the kiln exit gas stream. The cleaned gas tends to pass on the kiln side of the riser duct for a short time. Figure 17. Show the desired quench chamber position and fig 18 indicates the desired operating parameters for a kiln gas bypass quench chamber. In our experience a quench chamber operated with these parameters will not product any build-ups and will operate with no dust in the bottom of the chamber.

Kiln gas bypass dust collector material contains 0.520 per cent of clinker. 20-25 per cent SO₃, and 4.5-5.0 percent K₂O. lf the percentage of Sulphur as SO₃ is less. for example, 16 percent, the bypass system is taking too much dust from the kiln riser, etc.

There are always answers to problems with rings and build - ups. The solution is usually found when the attitude of the operator is that "we cannot continue to live with this problem."

This paper was first presented by the author, Floyds C Hamilton, of Hamilton Technical Services Inc., Roanoke, Virginia, for the National Lime Association meeting, St Louis, Missouri, United State.